Remove old C++03 move emulation code.

base/move.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_MOVE_H_
 #define BASE_MOVE_H_
 
 #include "base/compiler_specific.h"
 
-// Macro with the boilerplate that makes a type move-only in C++03.
+// Macro with the boilerplate that makes a type move-only in C++11.
 //
 // USAGE
 //
 // This macro should be used instead of DISALLOW_COPY_AND_ASSIGN to create
 // a "move-only" type.  Unlike DISALLOW_COPY_AND_ASSIGN, this macro should be
 // the first line in a class declaration.
 //
 // A class using this macro must call .Pass() (or somehow be an r-value already)
 // before it can be:
 //
 //   * Passed as a function argument
 //   * Used as the right-hand side of an assignment
 //   * Returned from a function
 //
-// Each class will still need to define their own "move constructor" and "move
-// operator=" to make this useful.  Here's an example of the macro, the move
-// constructor, and the move operator= from the scoped_ptr class:
+// Each class will still need to define their own move constructor and move
+// operator= to make this useful.  Here's an example of the macro, the move
+// constructor, and the move operator= from a hypothetical scoped_ptr class:
 //
 //  template <typename T>
 //  class scoped_ptr {
-//     MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
+//    MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type);
 //   public:
-//    scoped_ptr(RValue& other) : ptr_(other.release()) { }
-//    scoped_ptr& operator=(RValue& other) {
-//      swap(other);
+//    scoped_ptr(scoped_ptr&& other) : ptr_(other.release()) { }
+//    scoped_ptr& operator=(scoped_ptr&& other) {
+//      reset(other.release());
 //      return *this;
 //    }
 //  };
 //
-// Note that the constructor must NOT be marked explicit.
-//
-// For consistency, the second parameter to the macro should always be RValue
-// unless you have a strong reason to do otherwise.  It is only exposed as a
-// macro parameter so that the move constructor and move operator= don't look
-// like they're using a phantom type.
-//
-//
-// HOW THIS WORKS
-//
-// For a thorough explanation of this technique, see:
-//
-//   http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor
-//
-// The summary is that we take advantage of 2 properties:
-//
-//   1) non-const references will not bind to r-values.
-//   2) C++ can apply one user-defined conversion when initializing a
-//      variable.
-//
-// The first lets us disable the copy constructor and assignment operator
-// by declaring private version of them with a non-const reference parameter.
-//
-// For l-values, direct initialization still fails like in
-// DISALLOW_COPY_AND_ASSIGN because the copy constructor and assignment
-// operators are private.
-//
-// For r-values, the situation is different. The copy constructor and
-// assignment operator are not viable due to (1), so we are trying to call
-// a non-existent constructor and non-existing operator= rather than a private
-// one.  Since we have not committed an error quite yet, we can provide an
-// alternate conversion sequence and a constructor.  We add
-//
-//   * a private struct named "RValue"
-//   * a user-defined conversion "operator RValue()"
-//   * a "move constructor" and "move operator=" that take the RValue& as
-//     their sole parameter.
-//
-// Only r-values will trigger this sequence and execute our "move constructor"
-// or "move operator=."  L-values will match the private copy constructor and
-// operator= first giving a "private in this context" error.  This combination
-// gives us a move-only type.
-//
-// For signaling a destructive transfer of data from an l-value, we provide a
-// method named Pass() which creates an r-value for the current instance
-// triggering the move constructor or move operator=.
-//
-// Other ways to get r-values is to use the result of an expression like a
-// function call.
-//
-// Here's an example with comments explaining what gets triggered where:
-//
-//    class Foo {
-//      MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue);
-//
-//     public:
-//       ... API ...
-//       Foo(RValue other);           // Move constructor.
-//       Foo& operator=(RValue rhs);  // Move operator=
-//    };
-//
-//    Foo MakeFoo();  // Function that returns a Foo.
-//
-//    Foo f;
-//    Foo f_copy(f);  // ERROR: Foo(Foo&) is private in this context.
-//    Foo f_assign;
-//    f_assign = f;   // ERROR: operator=(Foo&) is private in this context.
-//
-//
-//    Foo f(MakeFoo());      // R-value so alternate conversion executed.
-//    Foo f_copy(f.Pass());  // R-value so alternate conversion executed.
-//    f = f_copy.Pass();     // R-value so alternate conversion executed.
-//
-//
-// IMPLEMENTATION SUBTLETIES WITH RValue
-//
-// The RValue struct is just a container for a pointer back to the original
-// object. It should only ever be created as a temporary, and no external
-// class should ever declare it or use it in a parameter.
-//
-// It is tempting to want to use the RValue type in function parameters, but
-// excluding the limited usage here for the move constructor and move
-// operator=, doing so would mean that the function could take both r-values
-// and l-values equially which is unexpected.  See COMPARED To Boost.Move for
-// more details.
-//
-// An alternate, and incorrect, implementation of the RValue class used by
-// Boost.Move makes RValue a fieldless child of the move-only type. RValue&
-// is then used in place of RValue in the various operators.  The RValue& is
-// "created" by doing *reinterpret_cast<RValue*>(this).  This has the appeal
-// of never creating a temporary RValue struct even with optimizations
-// disabled.  Also, by virtue of inheritance you can treat the RValue
-// reference as if it were the move-only type itself.  Unfortunately,
-// using the result of this reinterpret_cast<> is actually undefined behavior
-// due to C++98 5.2.10.7. In certain compilers (e.g., NaCl) the optimizer
-// will generate non-working code.
-//
-// In optimized builds, both implementations generate the same assembly so we
-// choose the one that adheres to the standard.
-//
 //
 // WHY HAVE typedef void MoveOnlyTypeForCPP03
 //
 // Callback<>/Bind() needs to understand movable-but-not-copyable semantics
 // to call .Pass() appropriately when it is expected to transfer the value.
 // The cryptic typedef MoveOnlyTypeForCPP03 is added to make this check
 // easy and automatic in helper templates for Callback<>/Bind().
 // See IsMoveOnlyType template and its usage in base/callback_internal.h
 // for more details.
 //
 //
-// COMPARED TO C++11
-//
-// In C++11, you would implement this functionality using an r-value reference
-// and our .Pass() method would be replaced with a call to std::move().
-//
-// This emulation also has a deficiency where it uses up the single
-// user-defined conversion allowed by C++ during initialization.  This can
-// cause problems in some API edge cases.  For instance, in scoped_ptr, it is
-// impossible to make a function "void Foo(scoped_ptr<Parent> p)" accept a
-// value of type scoped_ptr<Child> even if you add a constructor to
-// scoped_ptr<> that would make it look like it should work.  C++11 does not
-// have this deficiency.
-//
-//
-// COMPARED TO Boost.Move
-//
-// Our implementation similar to Boost.Move, but we keep the RValue struct
-// private to the move-only type, and we don't use the reinterpret_cast<> hack.
-//
-// In Boost.Move, RValue is the boost::rv<> template.  This type can be used
-// when writing APIs like:
-//
-//   void MyFunc(boost::rv<Foo>& f)
-//
-// that can take advantage of rv<> to avoid extra copies of a type.  However you
-// would still be able to call this version of MyFunc with an l-value:
-//
-//   Foo f;
-//   MyFunc(f);  // Uh oh, we probably just destroyed |f| w/o calling Pass().
-//
-// unless someone is very careful to also declare a parallel override like:
-//
-//   void MyFunc(const Foo& f)
-//
-// that would catch the l-values first.  This was declared unsafe in C++11 and
-// a C++11 compiler will explicitly fail MyFunc(f).  Unfortunately, we cannot
-// ensure this in C++03.
-//
-// Since we have no need for writing such APIs yet, our implementation keeps
-// RValue private and uses a .Pass() method to do the conversion instead of
-// trying to write a version of "std::move()." Writing an API like std::move()
-// would require the RValue struct to be public.
-//
-//
 // CAVEATS
 //
 // If you include a move-only type as a field inside a class that does not

[danakj, 2015/10/15 23:35:06]

I think this is not true with the new macro cuz the private copy constructor is
const type& not type& now. WDYT?

[dcheng, 2015/10/16 00:40:01]

Yeah, I think we shouldn't need this anymore. Removed.

I also mark them as deleted, because why not.

[danakj, 2015/10/19 21:49:24]

+1

 // explicitly declare a copy constructor, the containing class's implicit
 // copy constructor will change from Containing(const Containing&) to
 // Containing(Containing&).  This can cause some unexpected errors.
 //
 //   http://llvm.org/bugs/show_bug.cgi?id=11528
 //
 // The workaround is to explicitly declare your copy constructor.
 //
-#define MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \
- private: \
-  struct rvalue_type { \
-    explicit rvalue_type(type* object) : object(object) {} \
-    type* object; \
-  }; \
-  type(type&); \
-  void operator=(type&); \
- public: \
-  operator rvalue_type() { return rvalue_type(this); } \
-  type Pass() WARN_UNUSED_RESULT { return type(rvalue_type(this)); } \
-  typedef void MoveOnlyTypeForCPP03; \
- private:
+#define MOVE_ONLY_TYPE_FOR_CPP_03(type) \

[dcheng, 2015/10/13 21:03:09]

We should probably rename these macros, but I'd like to defer that to a separate
patch, once we're not using the old C++03 emulation anymore.

[danakj, 2015/10/15 23:35:06]

Or maybe make a base::move() and delete the macro.

[dcheng, 2015/10/16 00:40:01]

That's an option too, but I think that would involve a bunch of #include
juggling that I don't really want to deal with, especially since we'll have to
change the include again when we switch to std::move.

+  MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type)
 
 #define MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type) \
  private: \
   type(const type&); \
   void operator=(const type&); \
  public: \
   type&& Pass() WARN_UNUSED_RESULT { return static_cast<type&&>(*this); } \
   typedef void MoveOnlyTypeForCPP03; \
  private:
 
 #define TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type) \
  public: \
   type&& Pass() WARN_UNUSED_RESULT { return static_cast<type&&>(*this); } \
  private:
 
 #endif  // BASE_MOVE_H_